Film-forming powder, film forming method, and film-forming powder preparing method

ABSTRACT

A film-forming powder containing a rare earth oxyfluoride has an average particle size D50 of 0.6-15 μm, a total volume of ≤10 μm pores of 0.51-1.5 cm 3 /g as measured by mercury porosimetry, and a BET surface area of 3-50 m 2 /g is suitable for forming a dense film in high yields or deposition rates and high productivity. The film-forming powder having a greater pore volume can be prepared by forming a rare earth ammonium fluoride complex salt on surfaces of rare earth oxide particles to provide precursor particles, and heat treating the precursor particles at a temperature of 350 to 700° C.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2018-134243 filed in Japan on Jul. 17,2018, the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a film-forming powder suited for forming afilm as a protective coating in the interior of a semiconductormanufacturing apparatus or the like, and a method for preparing thesame. More particularly, this invention relates to a film-forming powdersuited for forming a film by the aerosol deposition method, and a methodfor preparing the same. This invention also relates to a method forforming a film from the film-forming powder by aerosol deposition.

BACKGROUND ART

The interior of a semiconductor dry etching apparatus is exposed tohighly reactive halogen or oxygen-based plasma. If members ofnon-plasma-resistant materials such as quartz glass, alumina andanodized aluminum are used as such, surface corrosion takes place toconcomitantly generate particles, which cause defects to microscopiccircuits on semiconductor. Therefore, the surface of a semiconductormanufacturing apparatus to be exposed to plasma is provided with aprotective coating for imparting corrosion resistance against the plasmaand protecting members of the apparatus.

One of such protective coatings is a coating of yttrium oxyfluorideexhibiting corrosion resistance against a wide variety of plasmas. Forexample, Patent Document 1 discloses that a dense, corrosion resistantprotective coating made of yttrium oxyfluoride powder and having acertain range of pore volume can be formed. While Patent Document 1describes that spraying, PVD, and aerosol deposition (AD) methods aresuitable for forming the protective coating, among them, the AD methodis successful in forming a protective coating having a smooth surfaceand causing least number of particles. Patent Document 2 describes thata coating of yttrium oxyfluoride formed by the AD method is dense ascompared with a coating formed by spraying.

CITATION LIST

-   Patent Document 1: JP-A 2017-150083 (US 2017/0342539 A1)-   Patent Document 2: KR 2011-0118939

DISCLOSURE OF INVENTION

As discussed above, a dense corrosion resistant film is obtained fromaerosol deposition of conventional yttrium oxyfluoride powder. Theaerosol deposition of conventional yttrium oxyfluoride powder has thedrawbacks of low yields (or deposition rates) of film formation and lowproductivity.

An object of the invention is to provide a film-forming powder suitedfor forming a film by aerosol deposition, especially for forming a densefilm in high yields (or deposition rates) by aerosol deposition; amethod for preparing the powder; and a method for forming a film on asubstrate by aerosol deposition.

Making investigations on a film-forming powder containing a rare earthoxyfluoride such as yttrium oxyfluoride, the inventors have found thatwhen a powder having a greater pore volume is used in aerosoldeposition, a dense film is formed on a substrate in high yields (ordeposition rates), and that the powder having a greater pore volume isprepared by forming a rare earth ammonium fluoride complex salt onsurfaces of rare earth oxide particles to provide precursor particles,and heat treating the precursor particles at a temperature of 350 to700° C.

With this preparation method, for example, a film-forming powdercontaining a rare earth oxyfluoride, and having an average particle sizeD50 of 0.6 to 15 μm, a total volume of pores having a diameter of up to10 μm in the range of 0.51 to 1.5 cm³/g as measured by mercuryporosimetry, and a specific surface area of 3 to 50 m²/g as measured bythe BET method is effectively prepared. Using this film-forming powder,a dense film contributing to high corrosion resistance can be formed inhigh yields (or deposition rates) by aerosol deposition.

In one aspect, the invention provides a film-forming powder containing arare earth oxyfluoride, and having an average particle size D50 of 0.6to 15 μm, a total volume of pores having a diameter of up to 10 μm inthe range of 0.51 to 1.5 cm³/g as measured by mercury porosimetry, and aspecific surface area of 3 to 50 m²/g as measured by the BET method.

Preferably, the fraction of particles having a particle size of up to0.3 μm is up to 0.5% by volume.

Preferably, the powder has an aspect ratio of 1.2 to 3.

Preferably, the powder has a dispersity index (b80) of up to 1.6, thedispersity index (b80) being determined according to the formula (1):

(D90−D10)/D50  (1)

wherein D10, D50 and D90 are cumulative 10%, 50% and 90% diameters involume basis particle size distribution, respectively.

Most often, the rare earth oxyfluoride is yttrium oxyfluoride.

In another aspect, the invention provides a method for forming a film,including the step of depositing the film-forming powder defined aboveon a substrate by the aerosol deposition method.

In a further aspect, the invention provides a method for preparing afilm-forming powder containing a rare earth oxyfluoride, including thesteps of forming a rare earth ammonium fluoride complex salt on surfacesof rare earth oxide particles to provide precursor particles, and heattreating the precursor particles at a temperature of 350 to 700° C.

Most often, the rare earth oxyfluoride is yttrium oxyfluoride, the rareearth oxide is yttrium oxide, and the rare earth ammonium fluoridecomplex salt is yttrium ammonium fluoride.

Advantageous Effects of Invention

By the preparation method of the invention, a film-forming powder havinga greater pore volume can be prepared. By using the film-forming powderof the invention and applying aerosol deposition, a dense film havinghigh corrosion resistance against halogen or oxygen-based plasma can beformed on a substrate in high yields (or deposition rates) and highproductivity. The film is suitable as a protective coating in theinterior of a semiconductor manufacturing apparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates a system for forming a film on asubstrate from a film-forming powder by the AD method.

FIG. 2 is a diagram showing an X-ray diffraction profile of the powderin Example 1 after drying and before heat treatment.

FIG. 3 is a diagram showing an X-ray diffraction profile of the powderin Example 1 after heat treatment.

FIG. 4 is a SEM photomicrograph of yttrium oxide powder used as thestarting material in Examples 4 to 6.

FIG. 5 is a SEM photomicrograph of the powder in Example 5 after heattreatment.

FIG. 6 is a diagram showing an X-ray diffraction profile of the powderin Example 5 after heat treatment.

DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the invention is a film-forming powder containing arare earth oxyfluoride. The rare earth oxyfluoride is a compoundconsisting of a rare earth element, oxygen, and fluorine, as representedby the compositional formula: REO_(x)F_((3-2x)) wherein RE is a rareearth element, and x is a positive number in the range 0<x≤1. Examplesof the rare earth oxyfluoride include REOF (corresponding to thecompositional formula wherein x=1), RE₅O₄F₇ (x=⅘), RE₆O₅F₈ (x=⅚), andRE₇O₆F₉ (x= 6/7). The rare earth oxyfluoride in the film-forming powdermay be a single compound or a mixture of two or more compounds.

Besides the rare earth oxyfluoride, the film-forming material maycontain other components such as rare earth oxides (e.g., RE₂O₃) andrare earth fluorides (e.g., REF₃). Preferably the film-forming powderconsists of a rare earth oxyfluoride. The presence or absence of rareearth oxyfluoride may be judged by analyzing a powder by X-raydiffractometry (XRD) to detect any rare earth oxyfluorides (such asREOF, RE₅O₄F₇, RE₆O₅F₈, and RE₇O₆F₉). Typically, Cu Kα ray as thespecific X-ray is used in the XRD.

When the film-forming powder contains another component such as rareearth oxide (e.g., RE₂O₃) or rare earth fluoride (e.g., REF₃), thepresence or absence of such oxide or fluoride may be judged by XRD. Whenthe film-forming powder consists of a rare earth oxyfluoride, only rareearth oxyfluoride is detected on XRD. When the film-forming powdercontains another component, peaks of rare earth oxide (e.g., RE₂O₃) orrare earth fluoride (e.g., REF₃) are detected. When the film-formingpowder contains another component in addition to rare earth oxyfluoride,the intensity of the maximum peak of the other component (where two ormore other components are contained, the total of intensities of theirmaximum peaks) is preferably up to 10%, more preferably up to 3% of theintensity of the maximum peak of rare earth oxyfluoride (where two ormore rare earth oxyfluorides are contained, the total of intensities oftheir maximum peaks). The peak intensity may be evaluated by peakheight. Also preferably, the film-forming powder is highly crystalline.Although it is acceptable that the powder contains a minor amount of anamorphous component, it is preferable that the powder consists ofsubstantially crystalline compounds.

Of the components constituting the film-forming powder, the componentsconstituting a raw material from which the film-forming powder isprepared, and the components constituting a film formed from thefilm-forming powder, the rare earth element (RE) is preferably one ormore elements selected from yttrium and Group 3 elements from La to Lu.Of these rare earth elements, preference is given to one or moreelements selected from yttrium (Y), samarium (Sm), gadolinium (Gd),dysprosium (Dy), holmium (Ho), erbium (Er), ytterbium (Yb), and lutetium(Lu). More preferably any of yttrium, samarium, gadolinium, dysprosiumand ytterbium is contained as the rare earth element. Even morepreferably the rare earth element consists of the majority (e.g., atleast 90 mol %) of yttrium and the balance of ytterbium or lutetium.Most preferably yttrium is the sole rare earth element.

Of the components constituting the film-forming powder, and thecomponents constituting a film formed from the film-forming powder, theinclusion of elements other than rare earth element, oxygen and fluorineis permissible as long as they are in the impurity amount. The contentsof Zr, Si, Al and Fe each are preferably up to 10 ppm (weight basis).

When the rare earth element is yttrium alone, the rare earth oxyfluorideis yttrium oxyfluoride. It is a compound consisting of yttrium, oxygen,and fluorine, as represented by the compositional formula:YO_(x)F_((3-2x)) wherein x is a positive number in the range 0<x≤1.Examples of the yttrium oxyfluoride include YOF (corresponding to thecompositional formula wherein x=1), Y₅O₄F₇ (x=⅘), Y₆O₅F₈ (x=⅚), andY₇O₆F₉ (x= 6/7). The yttrium oxyfluoride in the film-forming powder maybe a single compound or a mixture of two or more compounds. When therare earth element is yttrium alone, the rare earth oxide is yttriumoxide (e.g., Y₂O₃) and the rare earth fluoride is yttrium fluoride(e.g., YF₃).

Another embodiment of the invention is a method for preparing thefilm-forming powder, the method including the steps of (1) forming arare earth ammonium fluoride complex salt on surfaces of rare earthoxide particles to provide precursor particles, and (2) heat treatingthe precursor particles.

In step (1), for example, a powdery rare earth oxide (e.g., RE₂O₃) isdispersed in a solvent such as water or an organic solvent to form aslurry or dispersion of rare earth oxide. With stirring, ammoniumfluoride (e.g., NH₄F or NH₄HF₂) and optionally, a rare earth compound(rare earth-providing compound) other than the rare earth oxide, such asa rare earth nitrate (e.g., RE(NO₃)₃), rare earth chloride (e.g., RECl₃)or rare earth acetate (e.g., RE(CH₃COO)₃) are added to the dispersion.When no rare earth-providing compound is added, ammonium fluoride reactswith rare earth oxide in the surface portion of rare earth oxideparticles, yielding precursor particles having a rare earth ammoniumfluoride complex salt formed or precipitated on the surface of rareearth oxide (composite particles of rare earth oxide and rare earthammonium fluoride complex salt). When the rare earth-providing compoundis added, ammonium fluoride reacts with the rare earth-providingcompound, or the rare earth-providing compound and rare earth oxide inthe surface portion of rare earth oxide particles, yielding precursorparticles having a rare earth ammonium fluoride complex salt formed orprecipitated on the surface of rare earth oxide. The ammonium fluorideand the rare earth-providing compound may be added in solid form or insolution form after dissolution in a solvent such as water or organicsolvent. The ammonium fluoride may also be acidic ammonium fluoride.

In step (1), a powdery rare earth oxide (i.e., rare earth oxideparticles) is preferably used. In view of step (2) of causing rare earthoxide to react with rare earth ammonium fluoride complex salt, it may becontemplated to use as the starting material a substance capable ofgenerating rare earth oxide via pyrolysis, such as rare earth carbonateor rare earth hydroxide. However, such a substance has the drawbacksthat fine particles are often contained, particles are likely to bebroken to generate fine particles during formation of rare earthammonium fluoride complex salt, and particles coalesce together intolarge particles during subsequent heat treatment. Also, when rare earthcarbonate is used, there arises the drawback that undecomposed carbonoriginating from the rare earth carbonate is left behind if the heattreatment temperature is below 500° C. For these reasons, it isadvantageous to use powdery rare earth oxide as the starting material.

The use of powdery rare earth oxide as the starting material is alsoadvantageous in that a powdery rare earth oxide (i.e., rare earth oxideparticles) of relatively narrow or sharp particle size distribution isreadily available. Also, when the film-forming powder containing rareearth oxyfluoride is prepared by the inventive method using powdery rareearth oxide as the starting material, the resulting film-forming powderhas a dispersity index (b80) equal to or lower than that of powdery rareearth oxide. This means that when a powdery rare earth oxide having anarrow particle size distribution (or low dispersity index (b80)) isused, a film-forming powder having a narrower particle size distribution(or lower dispersity index (b80)) is obtained. When it is desired toobtain a film-forming powder having a dispersity index (b80) in therange defined later, for example, the powdery rare earth oxide shouldpreferably have a dispersity index (b80) of up to 2.5, more preferablyup to 2.3, even more preferably up to 2, because the particle sizedistribution becomes narrower as a result of conversion to thefilm-forming powder. The lower limit of dispersity index (b80) of thepowdery rare earth oxide is typically at least 0.7

When a film-forming powder containing a rare earth oxyfluoride isprepared by the inventive method using a powdery rare earth oxide as thestarting material, the resulting film-forming powder has an averageparticle size D50 equal to or greater than that of the powdery rareearth oxide. When it is desired to obtain a film-forming powder havingan average particle size D50 in the range defined later, the powderyrare earth oxide should preferably have an average particle size D50 ofat least 0.2 μm, more preferably at least 0.4 μm, even more preferablyat least 0.6 μm, and up to 15 μm, more preferably up to 10 μm, even morepreferably up to 8 μm, because the particle size can become greater as aresult of conversion to the film-forming powder.

When a film-forming powder containing a rare earth oxyfluoride isprepared by the inventive method using a powdery rare earth oxide as thestarting material, the resulting film-forming powder has an aspect ratioequal to or slightly lower than that of the powdery rare earth oxide.When it is desired to obtain a film-forming powder having an aspectratio in the range defined later, the powdery rare earth oxide shouldpreferably have an aspect ratio of at least 1.2, more preferably atleast 1.4, even more preferably at least 1.5, most preferably at least1.7, and up to 3.5, more preferably up to 3, even more preferably up to2.3, because the aspect ratio can become slightly lower as a result ofconversion to the film-forming powder.

The amount of ammonium fluoride added is such that a ratio (B/A) of thenumber of moles (B) of fluorine in ammonium fluoride to the total numberof moles (A) of rare earth elements in the dispersion (only rare earthoxide when no rare earth-providing compound is added, or the sum of rareearth oxide and rare earth-providing compound when rare earth-providingcompound is added) matches with the rare earth element-oxygen-fluorinecomposition of rare earth oxyfluoride to be created in the film-formingpowder. For example, REOF is created from a setting: B/A=1, RE₅O₄F₇ iscreated from a setting: B/A=1.4, and a mixture of REOF and RE₅O₄F₇ iscreated from a setting: 1<B/A<1.4. Also, a mixture of rare earth oxide(RE₂O₃) and REOF is created from a setting: B/A<1, and a mixture ofRE₅O₄F₇ and rare earth fluoride (REF₃) is created from a setting:B/A>1.4. Accordingly, the ratio (B/A) may be in a range: 0.9≤B/A<1 whenthe product may contain a small amount of rare earth oxide (RE₂O₃), andin a range: 1.4<B/A≤1.6 when the product may contain a small amount ofrare earth fluoride. In order to produce the film-forming powderconsisting of rare earth oxyfluoride, the ratio (B/A) is preferably setin the range: 1≤B/A≤1.4.

In the embodiment when precursor particles are prepared as a dispersion,the dispersion is subjected to solid-liquid separation, typicallyfiltration, to separate the precursor particles from the dispersion,i.e., to recover solids. If necessary, this is followed by the step ofrinsing with water or an organic solvent, the step of drying at roomtemperature (20°) to 100° C., and/or the step of passing through ascreen to loosen or disintegrate coagulated particles. The thus obtainedprecursor particles are ready to step (2). In step (2), the precursorparticles are heat treated. The heat treatment of the precursorparticles (or composite particles) causes the rare earth oxide to reactwith the rare earth ammonium fluoride complex salt, yielding particlescontaining rare earth oxyfluoride.

In the prior art method for preparing particles containing rare earthoxyfluoride, for example, by mixing and reacting rare earth oxide withrare earth fluoride as described in Patent Document 1, heat treatment ata high temperature in excess of 700° C. is necessary to promote thereaction and bring it to completion because the rare earth fluoride isnot readily decomposed unless heating temperature is in excess of 700°C. The heat treatment at high temperature causes sintering betweenparticles and densification in the interior of particles (particleshrinkage due to close contact of crystallites), resulting in particleswhich are densified in their interior to restrain plastic deformation atthe crystallite level necessary for aerosol deposition. Therefore, theprior art method fails to produce a film-forming powder of particleswhich are only limitedly densified in their interior so as to allow forplastic deformation at the crystallite level necessary for aerosoldeposition. In addition, since the prior art method is accompanied withan enlargement of particle size caused by sintering between particles, apulverizing step is necessary to reduce the giant particle size back toa size suited for film formation. Since the pulverizing step generallyuses a mill of stainless steel or the like and milling media of hardceramics such as zirconia, alumina, silicon nitride and silicon carbide,typically ceramic beads, it is impossible to obtain a film-formingpowder having minimized contents of Zr, Si, Al and Fe each up to 10 ppm(weight basis).

In contrast, the inventive method involves the steps of formingcomposite particles of rare earth oxide and rare earth ammonium fluoridecomplex salt and heat treating the composite particles to form particlescontaining rare earth oxyfluoride. The rare earth ammonium fluoridecomplex salt starts decomposition at about 350° C. and readily reactswith the rare earth oxide. When the precursor particles of rare earthoxide having rare earth ammonium fluoride complex salt formed on thesurface thereof are heat treated, efficient reaction takes place fromthe state that the rare earth oxide is in close contact with the rareearth ammonium fluoride complex salt. Then rare earth oxyfluoride iscreated at low temperature, as compared with the prior art method. Theinventive method for preparing a film-forming powder is successful inproducing particles susceptible to plastic deformation at thecrystallite level necessary for aerosol deposition, while restrainingsintering between particles and densification of particle interior. Thepulverizing step is unnecessary. The inventive method is successful inpreparing a film-forming powder having a total volume of pores having adiameter of up to 10 μm of at least 0.51 cm³/g, especially having anaverage particle size D50 of at least 0.6 μm and up to 15 μm, a totalvolume of pores having a diameter of up to 10 μm in the range of atleast 0.51 cm³/g and up to 1.5 cm³/g, and a BET specific surface area ofat least 3 m²/g and up to 50 m²/g. The contamination of impurityelements from the milling media is avoided.

In step (1), the slurry of rare earth oxide preferably has aconcentration of at least 5% by weight, more preferably at least 10% byweight and up to 30% by weight, more preferably up to 25% by weight. Forthe reaction or aging in step (1), preferably the temperature is 10 to80° C., and the time is 1 to 16 hours.

For the heat treatment in step (2), the temperature is preferably up to700° C., more preferably up to 680° C., even more preferably up to 630°C. from the aspect of restraining sintering between particles anddensification of particle interior. Since the rare earth ammoniumfluoride complex salt undergoes decomposition at about 350° C., the heattreatment temperature may be at least 350° C., preferably at least 400°C., more preferably at least 450° C. Examples of the heat treatmentatmosphere include oxygen gas-containing atmosphere, nitrogengas-containing atmosphere, and inert gas atmospheres such as helium gasor argon gas. Since the atmosphere is for reaction to produce rare earthoxyfluoride, the oxygen gas-containing atmosphere, typically airatmosphere is preferred because carbon, nitrogen and hydrogenoriginating from the starting materials can be removed by oxidation,typically by firing. The reaction or firing time is preferably 1 to 10hours.

In the film-forming powder, when the rare earth element is yttriumalone, the rare earth oxyfluoride in the powder is yttrium oxyfluoride.When such film-forming powder is prepared, the rare earth oxide and rareearth ammonium fluoride complex salt used in step (1) are preferablyyttrium oxide and yttrium ammonium fluoride, respectively.

According to the invention, there is provided a film-forming powdersuited for forming a film by aerosol deposition. The average particlesize D50 of the film-forming powder is preferably at least 0.6 μm and upto 15 μm. The average particle size D50 is more preferably at least 0.7μm or up to 10 μm. The average particle size D50 designates a cumulative50% diameter (or median diameter) in a volume basis particle sizedistribution and is measured by the laser diffraction method,specifically the laser diffraction/scattering method. A powder having anaverage particle size D50 below the range contains the majority ofparticles with a small size. In the aerosol deposition process, whenpowder is injected from the nozzle toward a substrate in the vacuumchamber, turbulent flow is generated by rapid volume expansion ofaerosol so that more particles of small size are scattered outside thesubstrate, and less particles are deposited on the substrate. On theother hand, a powder having an average particle size D50 above the rangecontains the majority of particles with a large size which areunamenable to aerosol deposition. That is, the proportion of particlesamenable to aerosol deposition is low. There is a high probability thatupon impingement against the substrate, particles of a large size arebounced back due to excessive kinetic energy, or scrape away a deposit(or film) formed on the substrate. An average particle size D50 outsidethe range may lead to a lowering of efficiency or yield of filmformation.

Preferably the film-forming powder contains up to 0.5% by volume ofparticles having a particle size of up to 0.3 μm. More preferably, thefilm-forming powder is substantially free of particles having a particlesize of up to 0.3 μm (i.e., 0% by volume). The particle size refers to aparticle size in a volume basis particle size distribution and ismeasured by the laser diffraction method, specifically the laserdiffraction/scattering method. A film-forming powder having a fractionof particles having a particle size of up to 0.3 μm above the range hasthe risk that particles agglomerate together in the aerosol and aredifficult to disperse and suspend uniformly, leading to a lowering ofefficiency or yield of film formation.

In the film-forming powder, the total volume of pores having a diameterof up to 10 μm is preferably at least 0.51 cm³/g and up to 1.5 cm³/g.More preferably the total volume of pores having a diameter of up to 10μm is at least 0.55 cm³/g or up to 1 cm³/g. Herein, the total volume ofpores having a diameter of up to 10 μm is measured by mercuryporosimetry. In the measurement of pore diameter distribution by mercuryporosimetry, generally a cumulative pore volume distribution relative topore diameter is measured, from which the total volume of pores having adiameter of up to 10 μm is determined.

In the aerosol deposition, when particles injected into vacuum areimpinged against the substrate, plastic deformation on the crystallitelevel occurs, and thus the particles deposit densely to form a film.However, particles in which the total volume of pores having a diameterof up to 10 μm is below the range have a low probability of plasticdeformation owing to the dense interior of particle. There is theincreased probability that particles are not deposited on the substrateas they are bounced back by the substrate upon impingement against thesubstrate. From the aspect of susceptible plastic deformation, as longas the total volume of pores having a diameter of up to 10 μm is atleast 0.51 cm³/g, a greater total volume is advantageous. However, ifthe total volume of pores having a diameter of up to 10 μm exceeds therange, the powder has a low (bulk) density, which means that particlesare too light when the average particle size D50 is in the above-definedrange. Then turbulent flow may be generated by rapid volume expansion ofaerosol so that a more fraction of particles may be scattered outsidethe substrate, and less particles may be deposited on the substrate. Atotal volume of pores having a diameter of up to 10 μm outside the rangemay lead to a lowering of efficiency or yield of film formation.

The film-forming powder preferably has a specific surface area (asmeasured by the BET method) of at least 3 m²/g, more preferably at least6.5 m²/g, even more preferably at least 9 m²/g and up to 50 m²/g, morepreferably up to 40 m²/g. If the BET surface area of a powder is belowthe range, the probability that plastic deformation occurs uponimpingement of particles against the substrate is reduced owing to lowsurface energy, and the probability that particles are not deposited onthe substrate as they are bounced back by the substrate upon impingementagainst the substrate is increased. In preparing a powder having a BETsurface area beyond the range, heat treatment at relatively lowtemperature is necessary, and there is the possibility that the complexsalt which has not been fully reacted is left. In addition, the powderhaving a BET surface area beyond the range consists of particles with alow (bulk) density which are too light within the above-defined range ofaverage particle size D50, leaving the risk that owing to the turbulentflow generated by rapid volume expansion of aerosol, more particles maybe scattered outside the substrate, and less particles may be depositedon the substrate. Either case may lead to a lowering of efficiency oryield of film formation.

The film-forming powder preferably has an aspect ratio of at least 1.2,more preferably at least 1.4, even more preferably at least 1.5, andfurther preferably at least 1.7, and up to 3, more preferably up to 2.3.The aspect ratio of the film-forming powder indicates a ratio of length(length along major axis) to width (width along minor axis, e.g., widthperpendicular to the length direction) of a particle and may bedetermined, for example, by taking an electron photomicrograph of 1,000×to 10,000× magnification, measuring the width and length of discreteparticles, and computing a ratio, for example, an average ratio of about100 or more particles. If the aspect ratio of a powder is below therange, the probability that plastic deformation occurs upon impingementof particles against the substrate is reduced owing to low surfaceenergy, and the probability that particles are not deposited on thesubstrate as they are bounced back by the substrate upon impingementagainst the substrate may be increased. If the aspect ratio of a powderis beyond the range, porosity may increase as plastic deformation occursin a distorted fashion. An aspect ratio outside the range may lead to alowering of efficiency or yield of film formation.

The film-forming powder preferably has a dispersity index (b80) of up to1.6, more preferably up to 1.5. The dispersity index (b80) is determinedaccording to the formula (1):

(D90−D10)/D50  (1)

wherein D10, D50 and D90 are cumulative 10%, 50% and 90% diameters involume basis particle size distribution, respectively, as measured bythe laser diffraction method, specifically the laserdiffraction/scattering method. As the dispersity index (b80) becomesgreater, the particle size distribution becomes broader, and thefractions of small size particles and large size particles areaccordingly increased. In aerosol deposition, small size particles andlarge size particles give rise to the above-mentioned problems. Afilm-forming powder having a dispersity index (b80) beyond the range,which has more fractions of small size particles and large sizeparticles, may lead to a lowering of efficiency or yield of filmformation. The lower limit of dispersity index (b80) of the film-formingpowder is typically at least 0.7.

The method for forming a film using the film-forming powder may includespraying, physical vapor deposition (PVD), and aerosol deposition. Thefilm-forming powder of the invention is especially effective when a filmis formed on a substrate by aerosol deposition, because a film having asmooth surface and causing least number of particles can be formed. Theresulting film is suited as a protective coating inside a semiconductormanufacturing apparatus or the like. Examples of the substrate includealuminum, nickel, chromium, zinc and alloys thereof, alumina, aluminumnitride, silicon nitride, silicon carbide and quartz glass, of whichmembers of the semiconductor manufacturing apparatus are made. The filmor coating thus formed preferably has a thickness of at least 2 μm, morepreferably at least 5 μm, and up to 50 μm, more preferably up to 30 μmalthough the thickness is not particularly limited.

When a film is formed by the step of depositing the film-forming powderon a substrate by the aerosol deposition method, the film-forming systemand conditions may accord with well-known system and conditions. Oneexemplary film-forming system is illustrated in FIG. 1 as including afilm-forming chamber 1 and a stage 2 which is disposed within thechamber 1 at its top such that the stage is movable in X-Y directions ina two-dimensional direction, specifically horizontal direction. Asubstrate S is mounted to the lower side of the stage 2. The systemfurther includes a vacuum pump 3 in fluid communication with the chamber1 through a conduit 31 for evacuating the chamber 1 to a reducedpressure. An aerosol generator 4 receiving a film-forming powder P is influid communication with the chamber 1 through a conduit 5. The conduit5 at its distal end in the chamber 1 is provided with a nozzle 51 whichis faced toward the substrate S. A carrier gas supply filled withcarrier gas 61 is in fluid communication with the aerosol generator 4through a conduit 6. A carrier gas 61 such as nitrogen gas is fed to theaerosol generator 4 to blow the film-forming powder afloat to createaerosol. The aerosol is fed through the conduit 5 and injected from thenozzle 51 toward the substrate S whereby a film or coating is depositedon the substrate S.

The film formed from the film-forming powder by aerosol deposition is afilm containing the rare earth oxyfluoride like the film-forming powder.There is formed a film reflecting the construction (compounds and theirratio) of the film-forming powder, that is, a film of substantially thesame constituents as the film-forming powder. When a film is formed fromthe inventive film-forming powder by film forming method, particularly,aerosol deposition, the resulting film is dense enough to have aporosity of up to 3% by volume, especially up to 1% by volume. Theporosity can be determined by observing a cross section of the filmunder SEM, performing image analysis and computing pore area.

EXAMPLES

Examples of the invention are given below by way of illustration and notby way of limitation.

Yttrium oxide powder was used as the starting material and afilm-forming powder was obtained therefrom. The particle sizedistribution (D10, average particle size D50, D90, and fraction ofparticles with a size of 0.3 μm or less) of the yttrium oxide powder andfilm-forming powder was measured by a particle size distributionmeasurement instrument (Microtrac MT3300 EXII by Nikkiso) relying on thelaser diffraction method, specifically the laser diffraction/scatteringmethod. A dispersity index (b80) was computed from the measurementresults according to formula (1). The pore size distribution of thefilm-forming powder was measured by a pore size distribution measurementinstrument (Auto Pore III by Micrometrics) relying on mercuryporosimetry. From the cumulative pore volume distribution relative topore diameter, the total volume (or cumulative volume) of pores with adiameter of 10 μm or less was computed. The BET specific surface area ofthe film-forming powder was measured by a full automatic surface areaanalyzer (Macsorb HM model-1280 by Mountech Co., Ltd.). The crystallinephase of the film-forming powder was analyzed by an X-ray diffractionanalyzer (X-Pert Pro MPD, CuKα ray, Malvern Panalytical Ltd.) in 2θrange of 5 to 70°. The aspect ratio of the film-forming powder wasdetermined by taking an electron photomicrograph of 1,000× to 10,000×magnification at a plurality of areas, measuring the width and length ofdiscrete particles, and computing an average of 200 particles. Thecontents of impurities (Zr, Si, Al, Fe) in the film-forming powder weremeasured by dissolving particles in an acid and analyzing by inductivelycoupled plasma (ICP) emission spectroscopy.

The thickness of a film formed from the powder was measured by an eddycurrent coating thickness tester (LH-300 by Kett Electric Laboratory).The porosity of the film was by observing and taking images of two viewfields of a cross section of the film under a SEM, and performing imageanalysis, and computing and averaging pore area of two view fields. Inparticular, the method was in conformity with ASTM E2109, the film wasembedded into resin to form a sample for SEM, then reflection electroncomposition images (COMPO images) were taken at magnification ratio of5,000×, and a gray value (threshold value) for binarization by densityslicing operation of 256 tones gray scale image was set to the valuethat is one tone lower than the largest tone in dark portionscorresponding to pore portions. The yield of film formation wasdetermined by dividing the weight of a film formed on the substrate bythe weight of the film-forming powder fed to the chamber of the filmdeposition system, that is, as percent deposition rate.

Examples 1 to 3

Yttrium oxide powder (average particle size D50=1.14 μm, dispersityindex b80=1.48, manufactured by Shin-Etsu Chemical Co., Ltd.), 1,129 g(5 mol, yttrium=10 mol), was mixed with 6.5 L of water and dispersed toform a dispersion or slurry. With stirring, 370.4 g (fluorine=10 mol) ofammonium fluoride (NH₄F) was rapidly added to the dispersion, which wasstirred and aged at 40° C. for 3 hours. In this duration, yttrium oxidereacted with ammonium fluoride, yielding precursor particles havingyttrium ammonium fluoride complex salt formed or precipitated on thesurface of yttrium oxide, i.e., composite particles of yttrium oxidewith yttrium ammonium fluoride.

Next, the dispersion was subjected to solid-liquid separation toseparate the precursor particles therefrom, i.e., to recover solids. Theprecursor particles were rinsed with water, dried at 80° C. for 16hours, and passed through a screen with an opening of 75 μm forloosening lightly coagulated particles. Thereafter, the precursorparticles were heat treated (or fired) in an electric furnace in airatmosphere at the temperature shown in Table 1 for 3 hours, yielding afilm-forming powder. FIG. 2 is an X-ray diffraction profile of thepowder in Example 1 after drying and before heat treatment. FIG. 3 is anX-ray diffraction profile of the powder in Example 1 after heattreatment.

A film was formed on a substrate by using the film deposition systemshown in FIG. 1, and moving the stage by reciprocating motion in ahorizontal direction, while effecting aerosol deposition of thefilm-forming powder. The deposition conditions are shown in Table 3.

Comparative Examples 1 and 2

A film-forming powder was obtained by the same procedure as in Example 1except that the heat treatment temperature was changed as shown inTable 1. Using the film-forming powder, a film was formed as in Example1.

Example 4

A film-forming powder was obtained by the same procedure as in Example 1except that the starting material was changed to yttrium oxide powder(average particle size D50=3.92 μm, dispersity index (b80)=1.90,manufactured by Shin-Etsu Chemical Co., Ltd.) and the heat treatmenttemperature was changed as shown in Table 1. Using the film-formingpowder, a film was formed as in Example 1. FIG. 4 is a SEM image of theyttrium oxide powder used as the starting material.

Example 5

A film-forming powder was obtained by the same procedure as in Example 1except that the amount of ammonium fluoride (NH₄F) was changed to 518.5g (14 mol). Using the film-forming powder, a film was formed as inExample 1.

FIGS. 5 and 6 are a SEM image and an X-ray diffraction profile of thepowder after heat treatment, respectively.

Example 6

Yttrium oxide powder (average particle size D50=3.92 μm, dispersityindex (b80)=1.90, manufactured by Shin-Etsu Chemical Co., Ltd.), 1,581 g(7 mol, yttrium=14 mol), was mixed with 10 L of water and dispersed toform a dispersion or slurry. With stirring, 10.0 L (yttrium=6 mol) of a0.6 mol/L yttrium nitrate aqueous solution and 10.0 L (fluorine=20 mol)of a 2.0 mol/L ammonium fluoride (NH₄F) aqueous solution were added tothe dispersion over 5 hours. In this duration, yttrium oxide and yttriumnitrate reacted with ammonium fluoride, yielding precursor particleshaving yttrium ammonium fluoride complex salt formed or precipitated onthe surface of yttrium oxide, i.e., composite particles of yttrium oxidewith yttrium ammonium fluoride.

Next, the dispersion was subjected to solid-liquid separation toseparate the precursor particles therefrom, i.e., to recover solids. Theprecursor particles were rinsed with water, dried at 80° C. for 16hours, and passed through a screen with an opening of 75 μm forloosening lightly coagulated particles. Thereafter, the precursorparticles were heat treated (or fired) in an electric furnace in airatmosphere at the temperature shown in Table 1 for 3 hours, yielding afilm-forming powder.

A film was formed on a substrate by using the film deposition systemshown in FIG. 1, and moving the stage by reciprocating motion inhorizontal direction, while effecting aerosol deposition of thefilm-forming powder. The deposition conditions are shown in

Table 3.

The average particle size D50, dispersity index (b80), fraction ofparticles with a diameter of 0.3 μm or less, BET surface area, totalvolume of pores with a diameter of 10 μm or less, aspect ratio,crystalline phase, and impurity content of the film-forming powders inExamples 1 to 6 and Comparative Example 2 are tabulated as Table 1. Thethickness, porosity and deposition rate of the films in Examples 1 to 6and Comparative Example 2 are tabulated as Table 2. It is noted that, inComparative Example 1, noticeable amounts of yttrium oxide and yttriumammonium fluoride complex salt were left behind, no analyses wereperformed, but the analysis of crystalline phase by X-raydiffractometry, and no film was formed. Thus, only the crystalline phaseof the film-forming powder is shown in Table 1.

TABLE 1 Fraction Yttrium of oxyfluoride diameter BET Firing particles≤0.3 μm surface Pore Aspect Crystalline Impurities temp D50 b80particles area volume ratio phase (ppm by weight) (° C.) (μm) (—) (vol%) (cm²/g) (cm³/g) (—) by XRD Zr Si Al Fe Example 1 450 1.49 1.19 0 34.30.615 1.7 YOF single phase <5 <5 <2 <2 2 630 1.53 1.37 0 15.3 0.579 1.8YOF single phase <5 <5 <2 <2 3 680 1.56 1.35 0 10.5 0.528 1.7 YOF singlephase <5 <5 <2 <2 Comparative 1 300 — — — — — — complex salt* + — — — —Example Y₂O₃ 2 750 1.71 1.44 0  3.2 0.475 1.7 YOF single phase <5 <5 <2<2 Example 4 500 6.37 1.23 0 20.3 0.592 2.0 YOF single phase <5 <5 <2 <25 500 6.78 1.13 0 10.5 0.643 2.3 Y₅O₄F₇ single phase <5 <5 <2 <2 6 5006.91 1.43 0 14.3 0.522 2.1 YOF single phase <5 <5 <2 <2 *complex salt:yttrium ammonium fluoride

TABLE 2 Film Deposition rate Thickness (μm) Porosity (%) (wt %) Example1 15 <1 0.28 Example 2 13 <1 0.25 Example 3 10 <1 0.19 ComparativeExample 1 — — — Comparative Example 2  4 <1 0.07 Example 4 15 <1 0.27Example 5 16 <1 0.30 Example 6  8 <1 0.17

TABLE 3 Substrate aluminum plate Substrate dimensions 30 mm × 30 mm × 3mm thick Nitrogen gas flow rate 5 L/min Pressure in chamber 0.1 kPaNozzle opening size 10 mm × 0.5 mm Nozzle-substrate stand-off distance 7mm Sweep speed 1 mm/sec Sweep distance 20 mm Sweep cycle 60 cycles

Japanese Patent Application No. 2018-134243 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A film-forming powder comprising a rare earth oxyfluoride, and havingan average particle size D50 of 0.6 to 15 μm, a total volume of poreshaving a diameter of up to 10 μm in the range of 0.51 to 1.5 cm³/g asmeasured by mercury porosimetry, and a specific surface area of 3 to 50m²/g as measured by the BET method.
 2. The powder of claim 1 wherein thefraction of particles having a particle size of up to 0.3 μm is up to0.5% by volume.
 3. The powder of claim 1, having an aspect ratio of 1.2to
 3. 4. The powder of claim 1, having a dispersity index (b80) of up to1.6, the dispersity index (b80) being determined according to theformula (1):(D90−D10)/D50  (1) wherein D10, D50 and D90 are cumulative 10%, 50% and90% diameters in volume basis particle size distribution, respectively.5. The powder of claim 1 wherein the rare earth oxyfluoride is yttriumoxyfluoride.
 6. A method for forming a film, comprising the step ofdepositing the film-forming powder of claim 1 on a substrate by theaerosol deposition method.
 7. A method for preparing a film-formingpowder comprising a rare earth oxyfluoride, the method comprising thesteps of: forming a rare earth ammonium fluoride complex salt onsurfaces of rare earth oxide particles to provide precursor particles,and heat treating the precursor particles at a temperature of 350 to700° C.
 8. The method of claim 7 wherein the rare earth oxyfluoride isyttrium oxyfluoride, the rare earth oxide is yttrium oxide, and the rareearth ammonium fluoride complex salt is yttrium ammonium fluoride.